Grounding switch

ABSTRACT

An improved grounding switch is provided capable of either an electrical drive, or manual actuation by means of a removable handle. The grounding is accomplished by the screw-driven linear motion of a cylindrical grounding contact, which moves into a stationary finger set attached to the main high-voltage conductor. The motor-driven grounding switch of the present application is particularly suitable for use with gas-insulated high-voltage transmission systems, and may, for example, be located at junction boxes provided at suitable locations.

United StateS Patent 1 [111 3,801,768 Meyer Apr. 2, 1974 GROUNDINGSWITCH Prima Examiner-Robert K. Schaefer 75 In e tor: effr R. Me er P nnHill., P'. ry 1 v n J y y e g d Assistant Examiner-William J. Smith [73]Assignee: Westinghouse Electric Corporation, Attorney, A t, r Firm-W. R.Crout Pittsburgh, Pa.

[22] Filed: Mar. 22, 1972 [5 ABSTRACT [211 Appl No: 237,079 A n improvedgrounding switch is provided capable of either an electncal dnve, ormanual actuation by means of a removable handle. The grounding is ac-U-S. Cl. R com lished the crew-driven linear motion of 3 cy. [51] Int.Cl. HOlh 31/24 lindrical grounding Contact, which moves into a Field ofSearch 200/48 43 148 G, tionary finger set attached to the mainhigh-voltage 200/148 B conductor.

The motor-driven grounding switch of the present [56] References cuedapplication is particularly suitable for use with UNITED STATES PATENTSgas-insulated high-voltage transmission systems, and 3,665,135 5 1972Boersma 200/163 may, for example, be located at junction boxes 3,700,84010/1972 Wilson et al.... 200/48 R rovided at uitable locations,2,955,182 10/1960 Caswell r 200/148 G D 3,562,465 2 1971 Turgeon..200/l63 9 (31811115, 11 Drawing Flgul'es lllllllllllllllllll l l l lPATENTED APR 2 I974 GROUNDING SWITCH CROSS-REFERENCES TO RELATEDAPPLICATIONS Reference is made to a related patent application filedMar. 27, 1972, Ser. No. 238,381, by Harvey Spindle, J. R. Meyer, and R.H. Hess, entitled Improved Switch and Mechanism Therefor, and assignedto the assignee of the instant application.

BACKGROUND OF THE INVENTION In recent years, there has come about ademand for a reduced-size substation, and this demand, on the part ofpublic utilities, has been met by gas-insulated substation equipment,such as set forth in US. Pats: No. 3,378,731, Whitehead; No. 3,348,001Upton et al; No. 3,356,798 McKinnon; No. 3,610,858 Gruber et al; No.3,599,041 Boersma et a]; No. 3,562,460 Koener.

The foregoing equipment significantly reduces the space required by thehigh-voltage side of substations rated, for example, ll5 KV through 345KV. The space reduction is accomplished by replacing the openbus andair-type bushings with gas-insulated bus filled, for example, with ahighly-insulating gas, such as sulfurhexafluoride (SP gas, at a pressuresay, for example, 45 p.s.i.g., and thereby permitting the movement ofelectrical equipment very closely together. This gasinsulated substationequipment has many advantages, among which are:

l. Significant reduction in space requirements both in land area andoverall height.

2. Added system reliability by eliminating the possibility ofphase-to-phase faults, lightning strokes within the system, orcontamination of insulators.

3. Reduced maintenance because the closed systemis isolated from itsenvironment.

4. Added personnel safety because all live parts are covered by groundedshields.

5. The gas-insulated modular approach has the additional advantage,because it provides the user with lower installation costs, whencompared with conventional, or other types of power systems.

The gas-insulated system, as briefly described above, has additionaldesign strategies, inasmuch as the highvoltage equipment is compressed,so that both the space required, and the total length of bus isminimized. The power transformers may be located on outside corners soas to be capable of ready removal, and the location of cable potheads isflexible, with the result that the system may be readily connected tooverhead lines.

As examples of the types of ratings for such gasinsulated transmissionsystems, reference may be made to the specification ratings, as setforth below:

650 310 Not applicable Symmetrical 3 Second Current Rating 47 kaMomentary Current 76 ka Switching Current Ratings Circuit breaker(maximum rated interrupting current) 50 ka Magnetizing current switch 35amps Isolator Ground switch Continuous Current Ratings Circuit breakerLoad break switch Magnetizing current switch No load switching only Noload switching only 2500 Amperes 2500 Amperes 2500 Amperes Isolator 2500Amperes Ground Switch Not applicable Bus 3000 Amperes 230 kv RatingsGeneral Ratings for MGT Systems SF. at 45 psig Rated maximum voltage 242BIL 900 60 HZ-one minute withstand 425 Chopped wave Not applicableSymmetrical 3 Second Current Rating 47 ka Momentary Current 76 kaSwitching Current Ratings Circuit breaker (maximum rated interruptingcurrent) 50 ka Magnetizing current switch 35 amps lsolator No loadswitching only Ground switch No load switching only Continuous CurrentRatings Circuit breaker Load break switch Magnetizing current switchIsolator Ground switch Bus 345 kv Ratings General Ratings for MGTSystems 2500 Amperes 2500 Amperes 2500 Amperes 2500 Amperes Notapplicable 3000 Amperes SF at 45 psig Rated maximum voltage 362Continuous Current Ratings Circuit breaker Load break switch Magnetizingcurrent switch 2500 Amperes 2500 Amperes 2500 Amperes Isolator 2500Amperes Ground switch Not applicable Bus 3000 Amperes It is desirable toprovide an improved grounding switch, which will permit the grounding ofthe highvoltage conductor, or bus, either at times when it is desirableto work upon the gas-insulated high-voltage system, or to deliberatelycreate a grounding fault and to trip remote circuit breakers. It isdesirable, in addition, to provide an improved switch of the foregoingtype, which may ground, for example, a 50 KVA fault for three secondswithout damage to the grounding switch.

SUMMARY OF THE INVENTION In accordance with a preferred embodiment ofthe present invention, a motor-driven grounding switch is provided tocause an extension and retraction of a movable grounding contact intoand out of engagement with the stationary grounding contact structure,associated with the high-voltage conductor, or high-voltage bus.Preferably, it is desirable to provide a motordriven actuation of thegrounding switch, and for emergency use, a portable collapsible handlemay be provided to enable manual grounding of the device.

Accordingly, it is a general object of the present invention to providean improved motor-driven grounding switch.

A more specific object of the present invention is the provision of animproved motor-driven grounding switch particularly adaptable for usewith a gasinsulated high-voltage transmission system.

Another object of the present invention is the provision of an improvedmotor-driven grounding switch, which also causes operation of auxiliaryswitches, and a proper indication of a position-indicator, such that theposition-indicator will be in a neutral position, other than when thegrounding switch is either in the fully-open or fully-closed positions.

Another object of the present invention is the provision of an improvedmotor-driven grounding switch, in which the same gear drive, whichactuates the movable grounding contact, also actuates a screw-drivenguideblock, which, through a cam actuator, effects a proper indicationof an externally-visible position-indicator.

Still a further object of the present invention is the provision of animproved grounding switch for a gasinsulated transmission system, inwhich the dielectric gas, such as sulfur-hexafluoride (SF gas, that isused in the main conduit system, is also used in the guide chamber forthe linearly-driven movable grounding contact, and surrounding thegas-insulated guide chamber is an outer chamber, preferably havingatmospheric air therein.

Another object is the provision of an improved grounding switch, as setforth in the immediately preceding paragraph, in which suitable heatingmeans are provided to prevent condensation within the surrounding airchamber.

Still a further object of the present invention is the provision of animproved grounding switch for a gasinsulated transmission system ofcompact dimensions, of rugged construction, and capable of fool-proofoperation, with the ultimate in safety provided for the maintenance man.I

Still a further object ofthe present invention is the provision ofanimproved grounding switch of the foregoing type, in which a removablecollapsible handle is 4 FIG. 7 is an inverted plan view, in section,taken substantially along the line VII-VII of FIG. 6;

FIG. 8 is a fragmentary vertical elevational view taken substantiallyalong the line VIII-VIII of FIG. 6;

FIG. 9 is an inverted plan view taken substantially along the line IX-IXof FIG. 8, illustrating the collapsed and storage position of thecollapsible handle;

FIG. 10 is a view, somewhat similar to that of FIG. 6, but showing thecollapsible handle extended, and in position to manually effect movementof the grounding switch; and,

FIG. 11 is a view of the collapsible handle in its extended position,ready for insertion into the handlestorage compartment of thegrounding-switch housing.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention hasparticular application to a new line of equipment 1 involvinggas-insulated substations having gas-insulated components, and somewhatdiagrammatically illustrated in FIGS. 1 and 2 of the drawings.

FIG. 2 is a one-line diagram of the equipment 1 illustrated in FIG. I.It will be noted, from a consideration of FIGS. 1 and 2, that thehigh-voltage equipment 1 is arranged so that both the space required,and the total length of the gas-insulated bus 3 is minimized. The powertransformer 4 is located on an outside corner,

7 preferably, so that it can be easily removed. The gasprovided, closeto the housing, for the grounding switch, and capable of being used inplace of the motordriven drive for emergency use. I

Further objects and advantages will readily become apparent upon readingthe following specification, taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat diagrammatic viewof a gasinsulated substation, showing the general environment for oneapplication of the improved grounding switch of the present invention;

FIG. 2 is a one-line diagram for the gas-insulated substation of FIG. 1;

FIG. 3 illustrates a junction box in section and a por-' insulated bus 3is attached directly to the transformer bushing minimizing area andheight required. The location of the cable pothead is flexible. In thegas-insulated system 1, as illustrated in FIGS. 1 and 2, it is chosen tominimize the length of the SF bus 3. If a lightning arrester 5 islocated ateach pothead 7, an arrester 5 is not required at thetransformer 4.

It will be noted that the gas-insulated system 1 of FIG. 1 can beconnected to overhead lines. However, the air clearances, required byincoming lines, will somewhat enlarge the total area required by thesystem 1, and will require additional SF bus 3. i

The gas-insulated transmission system 1 illustrated in FIG. 1 is a lineof equipment, which will significantly reduce the space required by thehigh-voltage side of substations rated 1 I5 KV through 345 KV. The spacereduction is accomplished by replacing the open bus and air bushings,commonly used, with gas-insulated bus 3 filled with sulfur-hexafluoride(SF gas, for example, at 45 psig (at F), and moving the electricalequipment as close together as possible.

The use of gas-insulated transmission systems 1 offer many advantages.The use of the system offers several advantages to the user, some ofthese are:

l. Significant reduction in space requirement both in land area andoverall height.

2. Added system reliability by eliminating the possibility ofphase-to-phase faults, lightning strokes within the system, orcontamination of'environment.

3. Reduced maintenance because of the closed system is isolated from itsenvironment.

4. Added personnel safety because all live parts are covered by groundedshields.

5. The modular approach, described in the following section, was chosenbecause it could provide the user with lower installation costs whencompared with conventional or other gas-insulated systems.

6. The system can be overbuilt to permit multiple use of land.

Generally, the equipment 1 includes a plurality of bus assembliesdetermined by the length that can generally be shipped. The typical buslength 3 will be, for example, 40 feet, and may consist of two ZO-feetlengths, with an epoxy spacer in each length. The ends of the bus 3 canbe connected to additional lengths of bus, or any functional member ofthe system. Expansion joints are located in each -foot bus section 3 toabsorb the 'maximum of 0.4 inches of expansion expected. As

stated, sulfur-hexafluroide (SF gas at 45 p.s.i.g., for example, fillsboth sheath 11 and bus conductor 3, and is free to move throughout theentire bus. The 45 p.s.i.g. pressure provides approximately the highestdielectric strength possible down to 40C without liquefaction,eliminating the need for auxiliary heat. The 45 p.s.i.g. pressure alsoeliminates the possibility of stable corona existing within the system1.

The present invention is particularly concerned with the motor-drivengrounding switch 13, illustrated in FIGS. 3-6 of the drawings. Withreference to these figures, it will be noted that there is provided agasinsulated bus, or high-voltage metallic conductor, designated by thereference numeral 3.

It will be noted that the high-voltage central conduc tor, orhigh-voltage bus 3, is spaced coaxially along an outer metallic groundedsheath, or cylinder 11 (FIG. 1) by a plurality of suitably spacedinsulating spacers (not shown). As will be obvious to those skilled inthe art, it may, at times, be necessary to sectionalize the gasinsulatedtransmission system 1,, and it may be desired to ground the centralhigh-voltage bus-conductor 3. Spaced at suitable intervals along thegas-insulated conduit system are a plurality ofjunction boxes 15,illustrated more clearly in FIGS. 3. and 4 of the drawings. Thesejunction boxes 15 constitute a suitable point for connections ofadditional components in the gasinsulated system 1. The outer case 17 ofthe junction box 15 is solidly bolted to the outer case of the adjacentdevice. Inside the junction box 15 is a set of shielded finger contacts19, which are mounted upon a tubular insulator 21. The innerhigh-voltage conductor 3 is free to move due to thermal expansion infinger contacts 19 mounted within the contact shield.

The junction box 15 is equipped to handle two bus runs 3 and a portableground. The junction box 15 can have up to six useful sides, which canbe used for a portable ground entrance, isolator switch entrance, or forup to five runs of bus 3.

The junction box 15 offers several advantages to both the designer andthe user. From the designers viewpoint, it offers a great deal offlexibility, and permits a modular approach to a system design. From theusers viewpoint, its compact design keeps the space require ments down,and eases field erection.

The junction box 15 has a pair of spaced finger assemblies 1.9, whichprovide expansion of the inner highvoltage bus 3. When initiallyinstalling the bus 3 in the system, the bus 3 is free to move throughthe center of the junction box 15. When the bus 3 is mounted at bothends, the keeper plate 23 is inserted, fixing the bus 3 in position.FIG. 4 illustrates the construction more clearly.

The present invention is particularly concerned with a motor-drivengrounding switch 13 to provide an effective ground for the gas-insulatedtransmission system l, in the presence of the maximum asymmetric faultcurrents that the system 1 can provide. A gearmotor 25 andscrew-mechanism 27 drive the grounding contact 29 in both directions atabout 0.6 inches per second for a total operating time of ten seconds. Avisible position-indicator 31 and auxiliary-switch system 33 are drivenby a gear 35 and screwdriver 36, that is independent of the motor-drivesystem. The positionindicator 31 has three distinct positions. Anintermediate position-indication is shown for all other positions thanfully-open or fully-closed. This has been accomplished by a cam-drivesystem 38 (FIG. 8), so that a positive indication is provided. Inaddition, a manual operating crank 40 is stored in the mechanism housing42, and can be provided with a padlock to positively lock the switch 13in the closed position.

FIG. 6 more clearly illustrates the construction of the screw-operatedmechanism 27. The electrical driving motor, designated by the referencenumeral 44, may be of the type ordinarily used in hand-drills, and may,for example, have a ipss ssd f QQQBBM-IM .dr ving shaft 46 of the motor44 is pinned to a driving gear 48, which meshes with a driven gear 50,which is pinned at 51 to the rotatable driving screw shaft 52. The motorpin is such that binding of any part of the ground switch mechanism willcause the motor gear pin to shear. Thus, no false status indication canresult. In

addition, the driving gear 48 also meshes .with another driven gear 35,which is pinned to an indicator-shaft 54, which effects linear motion ofa threaded guideblock 56. The guide-block 56 is interiorly threaded, andmoves back and forth in accordance with rotation of the'screw-shaft 54,which rotates in stationary bearings 59 and 60 (FIG. 6), one of thebearings 59 being supported in the mounting flange plate 62, and theother bearing 60 being provided by a mounting plate 64, in turn boltedto a support plate 66.

Consequently, when the driving motor 44 is energized, the three meshedgears 35, S0 and 48 rotate to effect linear opening and closing motionsof the tubular grounding contact 29 by a driven nut 68, which issecurely fastened to the movable grounding contact 29, and is threadedupon the threaded portion of the driving-screw shaft 52.

It will be noted that the grounding switch 13 comprises two compartments70, 7l, the inner one of which is in gas communication with theinsulating gas 72 utilized in the transmission system 1, namely,sulfurhexafluoride (SP gas, at a pressure say, for example, 45 p.s.i.

Externally of the guide chamber 73, constituted by the guide casting 75,is an outer air chamber 71 which is encased by an external mechanismhousing 42 having an accessopening 77 therein. A cover plate 79 (FIG. 9)covers the access opening 77, and, at times, permits the insertion of aremovable collapsible manually-operable handle 40, illustrated in FIG.11, and capable of emergency use.

As stated, the guide-block 56 moves a cam plate 81, more clearly shownin FIG. 8, to move the positionindicator lever 83 in a clockwisedirection, until the flat portion 810 of the cam plate 81 moves over theroller 85.

In more detail the rightward closing movement of the cam-plate 81 willforce the roller 85, affixed to the free end of the internalposition-indicator lever 83, toward the right, to the dotted-lineposition, as shown in FIG. 8, at which time the roller 85 is maintainedin this position by its engagement with the underside flat surface 81aof the cam-plate 81, until the vertical surface 81b of the cam-plate 81positively engages the roller 85, at which timethe grounding contacts22, 29 are closed. Further closing movement of the movable groundingcontact 29 will positively drive the roller 85 in a clockwise direction,as viewed in FIG. 8, thereby moving the external visible indicator lever31 in a clockwise direction to indicate the grounded position of thedevice 13.

It will, accordingly, be noted that the position- -indicator lever 31 isin its neutral position, as indicated 'lever 31 toward the open positionat all times. The indicator lever 31 is mounted upon a stub shaft 92,which extends through a Nylon sleeve 94 (FIG. 6), which prevents bindingshould snow or ice come between the metal bushing 96 and the stub shaft92. The coefficient of friction between the Nylon sleeve 94 and thesteel bushing 96 is such that no binding occurs, even though snow andice would tend to gather within the small clearance spaces therein.

Heaters 98 are provided within the outer air chamber 71 to preventinternal condensation of moisture within the outer air chamber 71. Itis, or course, only necessary to maintain the temperature within the airchamber 71 slightlyabove that of the outside atmosphere to prevent suchinternal condensation. The wires to the heater 98 may be attached to oneof three terminal blocks, designated by the reference numeral 100, andshown more clearly in FIG. 7 of the drawings.

FIG. 7 also illustrates more clearly the meshing of the three spur gears35, 48, 50 and also the storage compartment 102' for the collapsiblemanuallyoperable handle 40, illustrated in its collapsed position in FIG11' of the drawings.

FIG. 7 also shows a conduit entrance 104, through which wiring fromremote sources may extend. This wiring, of course, attaches to terminalblocks 100, which, in turn, carry the wiring to the driving motor 44,heaters 98, and also to the auxiliary switch, designated by thereference numeral 106, and illustrated more clearly in FIG. 8 of thedrawings.

Auxiliary Switch Operation The auxiliary switch, illustrated in FIGS. 7and 8 of the drawings, is actuated by a lever 107, which is engaged byadjustable studs 109, 110 carried by the movable guide-block 56. Asmentioned before, the guideblock 56 is threaded on a rotatable screwshaft 54, which is driven by the lower driven gear 35 of FIG. 6. Theauxiliary switch 106 is such that its operating lever 107 is biased to aneutral position, as indicated by the dotted lines 111 of FIG. 8. It isforcibly moved to either the opened or closed positions by positiveengagement with the adjustable studs 109, 110 carried by the movableguide-block 56. The auxiliary switch 106 may control various electricalcircuits, as well known by those skilled in the art.

It will be noted that a suitable seal 112 (FIG. 6) is provided extendingout the shaft opening 113 of the interiorly-located guide chamber 73;and a plurality of Teflon washers 114 are maintained in compression by ahelical spring 1 15, disposed within the annular recess 1 16 provided inthe guide-block 75. In addition, a split guide-sleeve 117 (FIG. 6) isinserted within the bore 1 18 of the guide-block 75, and serves as afurther guide for the linear movement of the movable tubular groundingcontact 29.

From the foregoing description, it will be apparent that an improvedgrounding switch 13 is provided, driven by a reversible motor 44, andcorrelated with a screw-mechanism 27 to positively actuateauxiliaryswitch operations and an externally-visible positionindicatordevcie 31. The structure is such that the external position-indicator 31is maintained in a neutral position until the grounding contacts 22, 29are actually either in the closed position, or in the almost fullyopenposition. This is brought about by the configuration of the cam-plate 81carried by the screw-driven guide-block 56.-

Also, the auxiliary switch 106, being biased to its neutral position, isonly opened or closed upon positive contact with the adjustablestud-boltsl09, carried by the guide-block 56.

MANUAL OPERATION The collapsible handle 40, illustrated more clearly inFIGS. 10 and 11, may be utilizedfor emergency use. Normally, it iscarried in a storage compartment 102, illustrated more clearly in FIG. 9of the drawings.

When it is to be used, a wing-nut 119 (FIG. 9) is re- Although there hasbeen illustrated and described a specific structure, it is to be clearlyunderstood that the same was merely for the purpose .of illustration,and that changes and modifications may readily be made therein by thoseskilled in the art, without departing from the spirit and scope of theinvention.

I claim as my invention:

1. A grounding switching comprising, in combination, a movable groundingcontact, a screw-shaft, means for rotating said screw-shaft by anelectrical motor, a nut fixedly secured adjacent one end of thegrounding contact, the screwshaft effecting linear opening and closingmotions of the grounding contact by threaded engagement with said nut,said electrical motor actuating a driving gear, a central gear fixedlysecured to the said screw-shaft, an auxiliary switch, and an auxiliarygear, said auxiliary gear being fixedly secured to a screw-shaft foreffecting operation of the auxiliary switch.

2. The combination according to claim 1, wherein a guide-block isthreaded onto the auxiliary screw-shaft and is moved thereby inaccordance with movement of the grounding contact, a cam-plate fixedlysecured to and movable with the movable guide-block, an indicatorpositioning device, and the cam-plate actuating the indicatorpositioning device.

3. A motor-driven grounding switch adaptable for use with agas-insulated transmission system comprising, in combination, meansdefining a guide-chamber having a linearly-movable tubular groundingcontact disposed therein, a driving screw-shaft disposed interiorly ofsaid tubular grounding contact, a driving motor, said motor driving saidscrew-shaft to effect opening and closing motions of the movable tubulargrounding contact, a position indicator, and an additional auxiliaryscrew-shaft rotates with rotation of the firstmentioned screw-shaft andeffects operation of the position-indicator.

4. The combination according to claim 3, wherein a movable guide-blockis threaded to said auxiliary nally movable within said hollow guidechamber, a nut (68) secured to said movable grounding contact, meanspreventing rotation of said nut, the region (70) interiorly of saidhollow guide chamber communicating with the gaseous region within saidgas-insulated transmission system, means (42) defining a second regionexternally of said hollow guide chamber (73) in communication with theatmospheric air, a motor disposed within said second region (71),gear-means disposed within said second region (71) and operativelyconnected with said nut (68) for causing the longitudinal motion thereofto effect thereby opening and closing motions of the movable groundingcontact.

6. The combination of claim 5, wherein a heater is provided within thesecond region (71) to prevent condensation therein.

7. The combination according to claim 5, wherein a second screw-shaft isconnected to said gear means and serves to operate a position-indicator.

8. The combination according to claim 7, wherein a cam-plate moves withsaid second screw-shaft and effects the operation of an auxiliaryswitch.

9. The combination according to claim 5, wherein said preventing meansincludes a longitudinal slot in the bore of said hollow guide chamber73).

1. A grounding switching comprising, in combination, a movable groundingcontact, a screw-shaft, means for rotating said screwshaft by anelectrical motor, a nut fixedly secured adjacent one end of thegrounding contact, the screw-shaft effecting linear opening and closingmotions of the grounding contact by threaded engagement with said nut,said electrical motor actuating a driving gear, a central gear fixedlysecured to the said screwshaft, an auxiliary switch, and an auxiliarygear, said auxiliary gear being fixedly secured to a screw-shaft foreffecting operation of the auxiliary switch.
 2. The combinationaccording to claim 1, wherein a guide-block is threaded onto theauxiliary screw-shaft and is moved thereby in accordance with movementof the grounding contact, a cam-plate fixedly secured to and movablewith the movable guide-block, an indicator positioning device, and thecam-plate actuating the indicator positioning device.
 3. A motor-drivengrounding switch adaptable for use with a gas-insulated transmissionsystem comprising, in combination, means defining a guiDe-chamber havinga linearly-movable tubular grounding contact disposed therein, a drivingscrew-shaft disposed interiorly of said tubular grounding contact, adriving motor, said motor driving said screw-shaft to effect opening andclosing motions of the movable tubular grounding contact, a positionindicator, and an additional auxiliary screw-shaft rotates with rotationof the first-mentioned screw-shaft and effects operation of theposition-indicator.
 4. The combination according to claim 3, wherein amovable guide-block is threaded to said auxiliary screw-shaft andcarries a cam-plate therewith, said cam-plate positively effectingindicating motion of the externally-mounted position-indicator, and theposition-indicator being biased to the open position.
 5. A motor-drivengrounding switch adaptable for use with a gas-insulated transmissionsystem comprising, in combination, a support plate (62) adaptable toclose an opening in said system, means defining a hollow guide chamber(73) secured to said support plate, a tubular movable grounding contact(29) longitudinally movable within said hollow guide chamber, a nut (68)secured to said movable grounding contact, means preventing rotation ofsaid nut, the region (70) interiorly of said hollow guide chambercommunicating with the gaseous region within said gas-insulatedtransmission system, means (42) defining a second region externally ofsaid hollow guide chamber (73) in communication with the atmosphericair, a motor disposed within said second region (71), gear-meansdisposed within said second region (71) and operatively connected withsaid nut (68) for causing the longitudinal motion thereof to effectthereby opening and closing motions of the movable grounding contact. 6.The combination of claim 5, wherein a heater is provided within thesecond region (71) to prevent condensation therein.
 7. The combinationaccording to claim 5, wherein a second screw-shaft is connected to saidgear means and serves to operate a position-indicator.
 8. Thecombination according to claim 7, wherein a cam-plate moves with saidsecond screw-shaft and effects the operation of an auxiliary switch. 9.The combination according to claim 5, wherein said preventing meansincludes a longitudinal slot in the bore of said hollow guide chamber73).